I understand that it is not downshifting. However, most cruise controls, if they cannot maintain the speed, they will kick out of cruise. This does not sound like your issue but just an FYI. Have you tried to see how low the speed will get before it might downshift?

 

My 96 is the same way. It is VERY reluctant to downshift out of O/D, even when speed drops below 60..... (from 75....) I think it is a PCM programming issue, more than anything else.

 

When you are driving, without the cruise, at 75, when does it kickdown for you?

 

I haven't tried that, I do know that it has gotten down to 62 ish and then by that point I was mad so I didn't let it go lower. It's never jumped out of cruise but after giving it a good push on the gas pedal it shifted out of overdrive and jumped down a gear then cruise control picked back up like everything was fine.

 

Does it downshift normally when not in cruise? Have you looked at or adjusted your TV cable?

 

Mine will downshift normally when not on cruise. Which leads me to the programming conclusion. Unfortunately, these trucks simply don't have enough power in fourth gear, to accelerate (or even maintain speed) up hill.

 

I haven't ever heard of the TV cable.. and I sure hope that the cruise isn't supposed to just not work properly.. I'll pay closer attention on the shifting while not in cruise but the handfuls of times I've driven it it's hour long trips or so on the highway

 

Just curious, do you you have the "wild vents" issue at all when this lugging / lack of downshifting is occurring?

I had the same problem you are experiencing - truck would hold top gear, locked up and refuse to downshift uphill with cruise on. It would sit there and hold high gear while dropping speed, to the point where it would be practically idling if I let it. I had the wild vents issue happening at the same time as well. It turned out to be a very small vacuum leak, a cracked rubber block-off on a T-fitting. Replaced that and it solved all my issues - downshifts correctly uphill after dropping ~5 km/h and no wild vents issues.

Assuming your TV is set correctly, double check all your vacuum lines - use a smoke machine if you can (cheap Halloween one or something).

 

Now THERE is an interesting idea. If there simply isn't enough vacuum for the cruise to pull the throttle open far enough, it won't downshift, and it will drop speed, even though the PCM is holding the valve on the cruise servo wide open.

Thanks Oxy. You have given me something else to look at.

 

If wild vents is talking about switching to the defrost in the doors or something like that I haven't even noticed it doing that but I can be on the lookout. I'll have to check all the hoses too for vacuum